Tool analysis device and method

ABSTRACT

The invention relates to a device and method for analysis of a tool ( 50 ) e.g. used on a machine tool. A tool detector ( 5 ) includes a light emitter ( 12 ) and a light receiver ( 34 ). Tool ( 50 ) when progressed into a beam ( 20 ) of light emitted from the emitter ( 12 ) will cause a signal from the receiver to change. Circuitry ( 32 ) includes a digital signal processor which processes the signal from the receiver and produces an output only if the signal conforms to a predetermined condition. Preferably this predetermined condition could be e.g. a characteristic shape of the signal from the receiver, a change in a value derived from a succession of such signals or a change in the minimum or maximum values of a succession of signals from the receiver.

This invention relates to a device and method usable for analysis of atool during its use with a machine tool, in particular but notexclusively for determination of the position of a tool.

Toolsetting devices for determining the position of a tool, which use abreak beam system are known. EP1050368 A1 describes in detail one systemwhich has a light transmitter and receiver. The transmitter produces abeam of light and a receiver has circuitry which produces a signal whenobstruction of the beam is detected. When a predetermined level of beamobstruction e.g. 50% is reached then the signal strength from thereceiver is reduced such that a trigger is produced. The trigger occurswhen a tool is present in the beam path.

A toothed cutting tool has to be rotated in order to find its cuttingdiameter. Usually it will have at least two teeth, one of which may betaller than the other and so that tooth will circumscribe a largerdiameter than the other(s). When the tool is brought into the beam paththe tallest tooth may be at any point and so the repeatability of thetrigger will vary. For example if a tool rotating at 1500 rpm is movingtowards the beam at 6 mm per minute then the feed per revolution wouldbe 6-1500 which equals 0.004 mm. So, the repeatability of the triggerwill be no better than 4 microns because the largest tooth may break thebeam anywhere in that feed per revolution distance. The feed perrevolution of the measurement limits the speed at which the tool can bedriven into the beam and so slows the detection rate. Detection speedneeds to be maximised for quick operation of the machine tool, whilstrepeatability needs to be maximised also, requiring slow feed rates.

One known way to increase detection speed is to move the tool quicklytoward the tool setting device and once detected to back off and thenmove in slowly to determine the tool's position. This procedure, whilstsaving some time, is still relatively time consuming.

According to a first aspect the present invention provides a toolanalysis device for use on a machine tool comprising a light emitter anda light receiver, the light receiver in use receiving light from theemitter and producing a signal indicative of the amount of light beingreceived, wherein the device further comprises a converter for providingdata having a numerical representation of the signal produced by thereceiver and comprising also a processor for processing that data andfor producing an output when the data conforms to a predeterminedcondition.

The processor may be a digital signal processor (DSP) operative toprocess the data according to an algorithm.

According to a second aspect the invention provides a method forprocessing an analogue signal resulting from light falling on a lightreceiver of a tool analysis device for use on a machine tool, comprisingthe steps of:

-   -   converting the analogue signal into data having a numerical form        which represents the signal; and    -   processing the data according to an algorithm.

Preferably the method provides a further step of:

-   -   producing an output signal when instructed by the algorithm when        the data conforms to a predetermined condition.

Preferably the method employs a DSP to process the data and thealgorithm is executed within the DSP.

According to both aspects the predetermined condition may be the dataobtained when the light falling on the light receiver is altered e.g. insuch a way that the tooth of a tool momentarily moves into and then outof the light falling in the light receiver, or a series of such eventsthat conform to a predetermined pattern. The pattern could be deviationsin the amount of that light such or a definable change in the magnitudeof those deviations, e.g. a decrease in magnitude from one deviation tothe next or a maximum followed by a minimum followed by another maximumin that magnitude.

Embodiments of the invention will now be described in detail withreference to the drawings, wherein:

FIG. 1 shows a break beam type tool detector and associated components;

FIG. 2 shows a schematic representation of elements of the break beamtool detector shown in FIG. 1;

FIG. 3 shows a flow diagram of an algorithm for use with the invention;

FIGS. 4,5,7 and 8 show simplified graphical representations of signalsproduced during use of the detector shown in FIG. 1; and

FIGS. 9-14 show graphs of actual output of a light receiver versus timeobtained during use of a detector of the type shown in FIG. 1.

FIG. 1 shows a break beam type tool detector 5. The detector is suitablefor use on a computer controlled machine tool (usually called a CNC)having a machine controller 56, a tool 50, a toolholder 52 and possiblyan automatic tool changer 54.

The tool detector 5 includes a laser light transmitter 12 (IR light isused in this instance), at transmitter portion 10, a light receiver 34at receiver portion 30 and a base 40 for mounting the transmitter andreceiver portions. A light beam 20 is created in use which can beobstructed by the tool 50. Additional circuitry 32 is used also forprocessing the signal issued by the light receiver 34. Whilst thecircuitry is shown in the receiver portion 30 some or all of it may belocated off the detector, e.g. as a PC interface card. FIG. 2 shows thecircuit 32 in more detail. A photodiode is used as the light detector34. The analogue signal from the photodiode is amplified at amplifier 31and is then sampled by an analogue to digital converter (A-D) 33. Thesample rate is approximately 45 KHz, but need not be that value. Asample is called for by the digital signal processor (DSP) and thisprocessor runs a continuously looping algorithm which calls for a sampleat each loop. The DSP is capable of parallel processing and so performsits tasks very quickly. The algorithm used performs the stepsillustrated in FIG. 3 and can be reset. In practice background noisewill be present and this will be sampled by the DSP. As the beam isobstructed e.g. by the tool, the signal from the receiver 34 will changeand thus will, in turn be sampled also. The algorithm can cause anoutput when the data conforms to a predetermined condition.

Prior to tool detection a calibration pin is detected by the detector. Apin, in this instance similar in size to the tool to be detected isbrought into the beam 20 by a program running in the CNC at a feed rateof approximately 4 mm/sec. The pin may be rotating or non-rotating. Thelight receiver output will follow a curve similar to that shown in FIG.4. That graph shows the receiver output in volts V, versus time. Astraight line n represents the nominal upper voltage of the receiveroutput with no beam obstruction. As the pin is brought into the beam anoscillation in output V is observed due to diffraction, then as less andless light from the beam falls on the receiver the voltage reduces alongcurve c and eventually drops to zero. In particular a noticeableincrease p in voltage is observed as the beam begins to be obstructed.The voltage during this calibration is processed by the DSP and turnedinto digital information representing the analogue output of the lightreceiver 34. The curve is stored as a digital representation in thememory of the DSP and is referred to below as the calibration curve c.More than one curve can be stored.

When the tool 50 is brought into the beam at about the same feed rate asthe pin, whilst it is rotating, it will have teeth which temporarilyobscure a part of the beam and these teeth cause voltage deviations s inthe signal from the receiver 34 as shown in FIG. 5.

It has been found that the minima and maxima of the deviations shown inFIG. 5 lie approximately on the calibration curve c. The graph shown inFIG. 5 is of an idealised form, and in practice far more interference ornoise is observed than has been shown. However, the voltage deviationsshown will be present despite the noise. For simplicity few deviationsare shown, however, in practice there may be many more than has beenshown because the tool will be rotating much faster than is exemplifiedand consequently many more interruptions to the beam will be observed.These additional deviations will also have minima and maxima which lieapproximately on the calibration curve c.

The deviations present will differ in shape depending on the tool typebut the minima and maxima will still lie on the calibration curve c.

Another set of deviations is shown in FIG. 6. This graph illustrates thevoltage deviations obtained when a two tooth cutting tool having onetooth taller than the other is brought into the beam. In this instancetwo sets of voltage deviations s and s′ are formed, lagging behind s.Each of these sets s and s′ has minima and maxima which lie on one ofthe respective calibration curves c or c′.

Yet another set of deviations is shown in FIG. 7. As well as thedeviations s this graph illustrates coolant drips etc d which occur whenthe beam 20 is obscured by material like machine coolant or swarf. Thesespurious signals d do not conform to any pattern or curve and occurrandomly.

In each of the graphs of FIGS. 5,6 and 7 when background noise andspurious signals d are ignored in favour of a recognisable set ofvoltage deviations s which are processed by the DSP.

Now, in this embodiment of the invention the voltage signals from thelight receiver are conditioned using the circuitry shown schematicallyin FIG. 2. Signal amplifier 31 amplifies the analogue signal and passesit to the A-D 33. The A-D in this example can operate at a 96 KHz samplerate but as illustrated in FIG. 3 is driven to collect each sample bythe downstream DSP 35. Once processed the sampled signals can be usede.g. to activate an output in the form of a trigger switch 37 e.g. toindicate that the edge of the tool has been found.

In this example the predetermined condition is the data obtained whenthe beam is obstructed by the tool.

The processing of the digitised signal by the DSP can be carried out ina number of ways. The DSP is an ideal device for carrying out suchprocessing because it is very fast and processes the data in real time,thus enabling short response times.

One method of processing the sampled data involves deriving a polynomiale.g. cubic expression for the calibration curve c shown in FIG. 4 andfitting, within tolerances, the minima and the maxima of the voltagedeviations s obtained in use of the device to that curve. If four minimaand maxima values (more could be used) have been identified which fit tothe curve within a predetermined time period then the DSP sends atrigger signal to switch 37 so that a 24 v skip signal can be issued tothe machine controller 56. The machine controller can use this signal todetermine the position of the tool's edges and thereby its effectivecutting path.

Alternatively an approximate straight line or polynomial can begenerated using the minima as points on that line. No calibrationcurve(s) is (are) required but rather, a threshold value can bepredetermined and when the straight line or polynomial line iscalculated to have crossed (or will cross when extrapolated) thatthreshold then a trigger signal can be generated. The calculationsnecessary for the line generation again can be performed in the DSP. Theaccuracy of the estimated threshold crossing point can be improved ifmore than the minimum number of points are used. A gradient method canbe used to determine minima or maxima and hence determine a thresholdvalue. The increase p can be used also to predict a threshold crossingif required.

Drips or similar obstructions to the beam may give rise to falsereadings during the use of the technique mentioned immediately above.There are a number of ways in which these false readings can be ignored.One such way is to look for periodic voltage deviations from thereceiver 34 and then to open time periods around the time when anotherdeviation is expected. Only in this period will digitising of the signaltake place. Thus, the chance of a drip occurring within that period willbe relatively small.

Another way to reject drips etc is to reject any voltage deviationminima or maxima whose values lie outside a band within which the nextsuccessive value for a predetermined curve would be expected.

The above examples for processing of sampled data can be used fornon-rotating or rotating tool detection including the detection of thelengths of rotating drills etc which, when brought into the beam on axismay appear to be non-rotating.

Another method of processing the sampled data from the edge of arotating tool is to look for voltage deviations which have a distinctivecharacter. A detail of one such voltage deviation is shown in FIG. 8.For certain signals, the deviation of the voltage when a cutting tooltooth obstructs the beam has a distinct shape caused by optical effects.The shape occurs only when a rotating part enters and leaves the beam.This distinctive shape can be detected by operating an algorithm in theDSP. This algorithm will look for the relatively high amplitude maximumh followed by a relatively high amplitude minimum s followed by anotherrelatively high amplitude maximum h. The minimum of that deviation andthe minimum of subsequent similar deviations can be identified as toothedge deviations and can thus be used to form a curve which can be fittedto a polynomial. No calibration curve(s) is (are) required for thismethod.

The characteristic shape mentioned above may be a min-max-min as well asthe max-min-max described.

A method of rejecting drips when detecting rotating tools is to ignorevoltage deviations which do not have the expected distinctive characteri.e. either min-max-min or max-min-max e.g. as shown in FIG. 8.

The techniques mentioned above for determining the position of a toolrely on the calculation of the minima and maxima of the voltagedeviations s which occur when a tooth obstructs the beam. There are anumber of well-known techniques for estimating the zero slope part of acurve from points on either side of minima or maxima which may be used.

FIG. 9 shows a graph of actual signals sampled from the output of a tooldetector of the type shown in FIG. 1. In this graph one square along theV axis represents 1 volt and one square along the time axis representsapproximately 25 msec.

A calibration pin has been brought into the beam and has caused thecharacteristic curve c which is shown in FIG. 4 in simplified form. Thenominal voltage n is 5.5 v and the receiver voltage drops to zero whenfully obstructed.

FIG. 10 shows a graph of actual signals sampled from the output of atool detector. In this graph a rotating tool (a 12 mm slot drill) isbrought into the detector beam and its teeth have produced the signal asthey obstruct the beam. The graph squares represent 1 volt and about 120msec. Superimposed on the graph is a the curve c which approximates tothe curve c in FIG. 9.

Various typical shapes of the data plots at points F11 to F14 along thecurve are shown in more detail in FIGS. 11 to 14 respectively. In eachof the graphs shown in FIGS. 11-14 one square represents 0.5 v and about5 msec.

In FIG. 12 the obstruction signal is still in its oscillatory phase andhas a min-max-min shape. In FIG. 12 a trough develops in the max andthus a min,max,min,max,min shape develops. In FIG. 13 that middle troughbecomes bigger. At FIG. 14 the trough is larger than the other signals.The signal in FIG. 14 is the max-min-max signal referred to above whichhas the distinctive shape that can be recognised and can bedistinguished from other spurious signals.

Tools which are brought into the beam along their axis of rotation e.g.drills brought in tip first also have this characteristic “c” shape.

Thus it can be seen that the voltage deviation that is detected by thealgorithm in the DSP develops as the tool is progressed into the beam.As mentioned above, there are many ways in which the sampled data can beused to determine a trigger point.

The max-min-max signal shown in FIG. 8 changes progressively as the toolmoves into the beam. The time t between the max peaks of the max-min-maxincreases as the tool progresses into the beam. The time t is shown alsoin FIG. 5 as t1,t2,t3,t4,t5 etc. This increase (or decrease if the toolis leaving the beam) can be determined by the DSP and used to determinewhere the tool is in relation to the beam. Thus a trigger threshold canbe set when a certain time t is reached for the recognisable max-min-maxsignal shown in FIG. 8. If the rate of increase of t is determined asthe tool is progressed then with a known velocity of tool relative tothe beam it will be possible to determine other tool characteristicssuch as tool geometry. If the change in t is determined for a giventravel of the tool then, with constant tool angular velocity, thediameter of the tool can be determined.

It has been found that the curve c shown in FIGS. 4-14 follows thedistinctive shape shown, even if the tool or obstruction in the beam isnot rotating as it enters the beam. Thus such a shape can be detected bythe algorithm in the DSP, e.g. by determining progressive min values (sin FIG. 8). It is not essential that a calibration curve (c in FIG. 4)is generated prior to tool analysis because the curve will follow arecognisable path of min values.

Thus it can be seen that there are a number of ways in which dataderived from receiver 34 can conform to a predetermined conditionindicative of a tool present in the beam 20 e.g.:

-   -   a number of repeated signals e.g. max,min,max which have a        characteristic profile;    -   the shape of curve c may conform to a calibration curve or may        have an expected shape;    -   the time t (FIG. 8) may vary such that a prediction of the        position of the tool can be made; or    -   successive peaks h (FIG. 8) may be encountered.

A DSP has been utilised in the embodiment described, however manyalternatives exist. Processing of the digitised signal could be carriedout using e.g: a Field-Programmable Gate Array, an application-specificintegrated circuit or a general-purpose microprocessor e.g. a PIC or aPC based system.

The tool detector is shown as a break beam type, but could be areflective type.

Any edge of a tool can be detected e.g. side or end. Usually acalibration curve will be generated for each edge which is to bedetected, but such a curve is not essential for edge detection.

The graphs show generally voltage output of a light receiver but anyother variable can be employed. The voltage etc may not drop fully tozero, particularly if a very small tool is to be detected and where onlya portion of the beam is obstructed.

Detection can be made when the tool is coming out of the beam as well asgoing into the beam. In such circumstances the effects detailed abovewill be reversed. Tools or other items to be detected need not berotating to be detected.

As described above the minima or maxima of voltage deviations ordeviations of variables can be fitted to a curve or simply used to plota line. In either instance a trigger point equivalent to a voltage (etc)threshold can be set.

1. A tool analysis device for use on a machine tool comprising a lightemitter and a light receiver, the light receiver in use receiving lightfrom the emitter and producing a signal indicative of the amount oflight being received, wherein the device further comprises a converterfor providing data having a numerical representation of the signalproduced by the receiver and comprising also a processor for processingthat data and for producing an output when the data conforms to apredetermined condition.
 2. A device as claimed in claim 1, wherein thepredetermined condition is data representing one or more occurrences ofan increase in the light from the emitter received at the receiver,followed by a decrease in that light followed by another increase.
 3. Adevice as claimed in claim 1, wherein the predetermined condition isdata representing a succession of decreases in the light from theemitter received at the receiver the minimum values of which conformsubstantially to a curve of a type expected by the processor.
 4. Adevice as claimed in claim 1, wherein the predetermined condition isdata representing the change in the time between occurrences of anincrease or decrease in the amount of the light from the emitterreceived at the receiver.
 5. A device as claimed in claim 1, wherein thepredetermined condition is data representing an increase in light fromthe emitter received at the receiver at a level above that expected fromthe emitter.
 6. A device as claimed in claim 1, wherein the processor isa digital signal processor operative to process the data continuouslyaccording to an algorithm.
 7. A method for processing an analogue signalresulting from light falling on a light receiver of a tool analysisdevice for use on a machine tool, comprising the steps of: convertingthe analogue signal into data having a numerical form which representsthe signal; and processing the data according to an algorithm.
 8. Amethod as claimed in claim 7 wherein the method further includes thestep of producing an output signal when instructed by the algorithm whenthe data conforms to a predetermined condition.
 9. A method as claimedin claim 8 wherein the predetermined condition is data representing oneor more occurrences of an increase in the light from the emitterreceived at the receiver, followed by a decrease in that light followedby another increase.
 10. A method as claimed in claim 8 wherein thepredetermined condition is data representing a succession of decreasesin the light from the emitter received at the receiver the minimumvalues of which conform substantially to a curve of a type expected bythe processor.
 11. A method as claimed in claim 8 wherein thepredetermined condition is data representing the change in the timebetween occurrences of an increase or decrease in the amount of thelight from the emitter received at the receiver.
 12. A method as claimedin claim 8 wherein the predetermined condition is data representing anincrease in light from the emitter received at the receiver at a levelabove that expected from the emitter.
 13. A device as claimed in claim2, wherein the processor is a digital signal processor operative toprocess the data continuously according to an algorithm.
 14. A device asclaimed in claim 3, wherein the processor is a digital signal processoroperative to process the data continuously according to an algorithm.15. A device as claimed in claim 4, wherein the processor is a digitalsignal processor operative to process the data continuously according toan algorithm.
 16. A device as claimed in claim 5, wherein the processoris a digital signal processor operative to process the data continuouslyaccording to an algorithm.